1. Field of the Invention
The present invention relates to a brush used in an electric generator, a commutator motor, and the like, and slidably contacted with their commutators, and a commutator motor having a brush device using the brush.
2. Description of the Related Art
The brushes used in the commutator motors and the like are roughly classified into carbon type and noble metal type. The carbon type brushes are generally used. The carbon type brush is formed by mixing powdered carbon as a main component with binding material, compressing and solidifying the mixture and sintering the solidified mixture at a high temperature. As the binding material, a synthetic resin is generally used. For example, a conventional brush used for the commutator motor installed in an electric air flower of a vacuum cleaner is composed of one type of carbon as a main component. In order to reduce sparks caused upon slidable contact between brushes and a commutator by increasing the value of resistance commutation, a carbon material as the main component of the conventional brush generally has a high resistance more than 30,000.mu..OMEGA. per square centimeter. Therefore, the value of current short-circuiting be%ween a plurality of commutator segments of the commutator through the brushes is reduced when the commutator segments contact the distal end surface of each brush, so that sparks generated between the brushes and the commutator segments are reduced.
The above-mentioned conventional brush, however, has the following problems. Since the brush has a high total electric resistance (about 0.2.OMEGA.) and hence has a high resistance loss of current (about 20 W per one brush), the operation efficiency of the commutator motor using the brushes is decreased.
In addition, since the resistance loss of the current is converted into heat, in the resin binded type brush using the synthetic resin binding material, a heat deterioration of the binding material tends to occur. In the commutator, a synthetic resin portion for insulating the commutator segments from each other is susceptible to the heat deterioration. Especially, the commutator motor used in the electric air flower of the vacuum cleaner tends to decrease in size and increase in input. With this tendency, the current density of the brush is increased. Since the brush has a high electric resistance, it tends to generate a great amount of heat. This shortens the service lives of the brush and the commutator. It is known that the flexural strength of the conventional resin binded type brush is abruptly reduced when its temperature exceeds 200.degree. C.
The service lives of the brush and the commutator can be prolonged if their sizes are increased to reduce the current density and improve heat radiation from their surfaces. With such increases in size, however, the axial length of the commutator motor is increased.